1. Field of the Invention
The present invention relates to a projection type green cathode ray tube (CRT) and, more particularly, to a projection type green light-emitting CRT which has a phosphor screen formed by a cerium-activated calcium sulfide phosphor. The present invention further relates to a method for manufacturing the phosphor screen and to a projection video device which includes the green CRT described above.
2. Description of the Prior Art
Projection video devices enlarge images on the CRT and project them on a large screen. High brightness CRTs used in these devices are called projection type CRT.
The projection video devices are mainly used to reproduce TV images for education and leisure. It is expected that high-density scanning technique (high resolution) of the screen is further improved in TV broadcasting and video systems for a variety of appli- cations.
In order to maximize brightness of an image reproduced on the large screen, electron beams are emitted on the phosphor screen of the projection type CRT with energy of more than 10 times the energy applied to a phosphor screen of a display color CRT. For this reason, the temperature of the phosphor screen is increased up to 150.degree. C. at maximum in the normal operation. However, brightness of the phosphor screen is generally decreased with an increase in the temperature of the phosphor screen.
When a white image is reproduced on the projection screen, using a projection color video device, about 70% of the total brightness is obtained by green color components. The phosphor screens of the green CRTs used in the conventional projection video devices are formed of manganese-activated zinc silicate or terbium-activated gadolinium oxysulfide phosphors. The former phosphor has a low fluorescent efficiency upon radiation with electron beams and is "burnt" by high electron energy, resulting in degradation in the quality of the phosphor screen. On the other hand, the latter phosphor has a high fluorescent efficiency upon radiation with electron beams. However, this fluorescent efficiency is significantly decreased with an increase in the temperature of the phosphor screen. For this reason, the faceplate of the CRT is cooled by air from the fan. However, this does not provide satisfactory effects. Color images become reddish after some time from the beginning of projection. Therefore, a contrast adjustment must be performed again, resulting in inconvenience.